This invention relates generally to surface inspection and characterization systems. More specifically, the invention relates to optical based, surface measuring systems in which a plurality of beams of light are directed onto a workpiece for reflection therefrom.
It is often necessary to measure the surface characteristics of painted, plated, polished or otherwise finished objects in the course of their manufacture. The visual appearance of a finished surface is a highly subjective determination; however, a number of parameters have been established in the art to quantify aspects of surface quality. Among the measurements which are commonly made are: distinctiveness of image (DOI), which is a measure of how clearly an object is reflected by a surface; gloss, which is a measurement of the dispersion with which light is reflected from a surface; and orange peel, which is a surface characteristic dependent upon the presence of a repeating pattern of surface features having a texture in the general size range of 0.5 to 1.0 millimeter, and which produce a texture appearing somewhat reminiscent of the skin of an orange. Other parameters which are sometimes measured include tension, which is a measurement of the clarity with which a projected grid pattern is reflected from a surface, and haze (also referred to as texture), which is a measurement of the clarity with which the image of an object is reflected from the surface. There is overlap in the quantities which are measured by these various determinations, and various manufacturers will rely upon different ones of these parameters for quality control purposes. In some instances, these surface quality parameters are assessed subjectively by comparing a finished surface with a series of standards. In other instances, various algorithms have been developed for calculating these parameters, based upon the reflection of light from the surface. For example, some particular techniques for the measurement of gloss and distinctiveness of image are disclosed in U.S. Pat. Nos. 4,761,676; 4,746,805 and 4,527,898.
In a manufacturing environment, automated systems are often utilized for quantifying the aforementioned parameters. Such systems require accuracy, ease of use, high speed operation, mechanical simplicity and low cost; however, these parameters are often mutually exclusive. Accuracy generally requires that a relatively large area of a workpiece be measured so as to produce a representative reading of its properties. However, using one single, large area light beam to read the workpiece produces errors, since the large beam will fail to read small area features and can also introduce sampling errors. For example, orange peel readings are very difficult to automate. Orange peel results from the presence of relatively small features, which repeat at differing frequencies. If a large area beam is used to measure orange peel, it will fail to read relatively small features; but, if a small diameter beam is utilized for orange peel measurements, it cannot provide an accurate reading unless it is scanned over a relatively large area. As a consequence, the prior art has typically raster scanned a measuring beam across a relatively large area of a workpiece in order to obtain surface quality measurements. Such scanning is slow, and requires complicated hardware.
The prior art has implemented a number of approaches to improving automated surface quality measurements, and various of these approaches are found in U.S. Pat. Nos. 5,570,183; 5,686,731 and 4,989,984. In some instances, multiple beams are employed for scanning a surface, and such approaches are shown in U.S. Pat. Nos. 4,265,545 and 4,110,047. Prior art systems tend to be expensive, mechanically complex and slow in operation. Accordingly, there is a need for an optical inspection system which, in addition to being accurate, is fast in operation and simple in construction. As will be explained in detail hereinbelow, the present invention provides a surface quality measuring system which utilizes a plurality of light sources segregated into two distinct groupings. Use of a number of sources simplifies the optical system of the device and minimizes sampling times. In addition, the system of the present invention utilizes a time multiplexed detection system which eliminates redundancy in parts and improves the reliability and operating speed of the device. These and other advantages will be apparent from the drawings, discussion and description which follow.
There is disclosed herein an apparatus for measuring the visual characteristics of a surface of a workpiece. The apparatus includes a first light source which is operative to direct at least one focused beam of light onto the workpiece for reflection therefrom and a second light source operative to direct at least one unfocused beam of light onto the surface of the workpiece for reflection therefrom, at a location separate from the location at which the first beam or beams impinge the workpiece. The system also includes a photo detector which produces a signal in response to illumination, means for directing the first and second sets of reflected beams of light onto the photo detector, and a multiplexer for causing the photo detector to sequentially detect each member of the first and second sets of reflected beams so that the photo detector produces a separate signal corresponding to each of the reflected beams. The system can further include a signal processor which processes data from the photo detector and generates readings of surface quality such as DOI, orange peel and gloss.
In particular embodiments, the first light source directs a plurality of focused beams of light onto the workpiece and the second source directs a single unfocused beam onto the workpiece. The light sources may comprise diode lasers, and in particular embodiments, the beams from the first source are directed onto the workpiece at the Brewster angle for that wavelength and workpiece material. Illumination at the Brewster angle maximizes surface reflection thereby avoiding interference from metal flakes and other such subsurface reflective bodies.
In another aspect of the present invention, a light source of the apparatus includes a plurality of separate light emitters, each light emitter operative to direct a single beam of light onto a different portion of the surface of a workpiece, at a common angle of incidence for reflection therefrom. In this embodiment, the light emitters are disposed in an angled relationship to one another so that the beams of light therefrom follow paths which are converging as they approach the surface. The paths meet at a convergence point following reflection. Most preferably, a photo detector is disposed at the convergence point, and in this manner, a single photo detector can receive and read all of the beams. Multiplexing of the beams allows for signal separation.